An image sensor is a semiconductor device for converting optical images into electrical signals. Image sensors may be classified into charge coupled device (CCD) image sensors and complementary metal oxide silicon (CMOS) image sensors (CIS).
During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion may also reduced, thereby resulting in a reduction in image quality.
Also, since a reduction in stack height may not be proportionate to a reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion may also be reduced due to diffraction of light called airy disk.
To overcome this limitation, methods including forming a photodiode using amorphous silicon (Si), or forming readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on the readout circuitry have been employed (referred to as a “three-dimensional (3D) image sensor”). The photodiode may be connected to the readout circuitry through a metal interconnection.
In the related art image sensor, a metal interconnection may be formed on the readout circuitry, and a wafer-to-wafer bonding may be performed such that the photodiode contacts the metal interconnection. However, it may be difficult to properly achieve a contact between the photodiode and the metal interconnection during the wafer-to-wafer bonding.
Moreover, in the related art image sensor, there may be difficulty in electrically isolating a contact for one color from a contact for another color in a stacked RGB photodiode configuration (e.g., containing red, green and blue photodiodes in a vertical arrangement).
Since both the source and the drain on opposite sides of the transfer transistor may be heavily doped with N-type impurities, a charge sharing phenomenon may occur. When a charge sharing phenomenon occurs, the sensitivity of an output image may be reduced and an image error may be generated. Also, because photocharges do not readily move between the photodiode and the readout circuitry, a dark current may be generated and saturation and/or sensitivity may be reduced.